Furnaces
Content
Module: 3
Lecture: 9 Sodium carbonate
Dr. N. K. Patel
Module: 3
Lecture: 9
SODIUM CARBONATE
INTRODUCTION
Sodium carbonate (Na2CO3) also known as washing soda or soda ash, is a
sodium salt of carbonic acid. Most commonly occurs as a crystalline heptahydrate,
which readily effloresces to form a white powder, the monohydrate. Sodium
carbonate is domestically well known as a water softener. It can be extracted from
the ashes of many plants. It is synthetically produced in large quantities from salt and
limestone in a process known as the Solvay process.
Soda ash is the most important high tonnage, low cost, reasonably pure,
soluble alkali available to the industries as well to the laboratory.
MANUFACTURE
Sodium carbonate is manufactured by following process.
1.
2.
3.
4.
Leblanc process.
Solvay‘s ammonia soda process.
Dual process (modified Solvay‘s process)
Electrolytic process.
1. Leblanc process
The process has only historical importance, because is now been replaced
completely by Solvay process or modified by Solvay process.
Raw materials
Basis: 1000kg Sodium carbonate (98% yield)
Common salt
= 1126kg
Sulfuric acid
= 945kg
Lime stone
= 963kg
Coke
= 463kg
Sources of raw material
Common salt can be obtained from sea water, salt lake and sub –soil water
as described in Module: 3, Lecture: 8.
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Lecture: 9 Sodium carbonate
Dr. N. K. Patel
Sulfuric acid can be obtained by contact process as described in Module: 4,
Lecture: 18
Lime stone is obtained from mineral calcite or aragonite, which can be used
after removal of clay, slit and sand (silica).
Reactions
NaCl + H2SO4
NaHSO4 + NaCl
Na2SO4 + 4C
Na2S + CaCO3
NaHSO4 + HCl
Na2SO4 + HCl
Na2S + 4CO
Na2CO3 + CaS
(Black ash sludge)
CaS + H2O + CO2
CaCO3 + H2S
CaS + H2S
Ca(HS)2
Ca(HS)2 + CO2 + H2O
CaCO3 + 2H2S
H2S + O
H2O + S
Manufacture
Water
NaCl
Flue
gases
Lime
Stone Coke
NaCl
HCl
Salt
Cake
Furnace
Concentrated
H2SO4
Furnace
Black ash
Rotary
Furnace
Pulverizer
Mixer
Mixer
Crusher
Water
Water
out
Tower
Na2CO3
Leaching
Open pan
Evaporator
Calcination
tower
Water
in
Cooler
Hot gas
Figure: Manufacturing of Sodium Carbonate by Lablance process
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Lecture: 9 Sodium carbonate
Dr. N. K. Patel
Block diagram of manufacturing process
Diagram with process equipment
Animation
Common salt is first mixed with the conc. H2SO4 in equivalent quantities and
heated in a cast iron salt cake furnace by flue gases from adjacent coal of fire.
NaHSO4 along with HCl gas is formed. HCl is passed to tower packed with coke and
is absorbed through a spray of water comes down in the tower. The paste of
NaHSO4 is taken out and heated to a high temperature on the hearth of a furnace
along with some more common salt. NaHSO4 is thus converted into sodium sulfate,
known as salt cake.
The salt cake is broken or pulverized, mixed with coke and limestone and
charged into black ash rotary furnace consisting of refractory lined steel shells. The
mass is heated by hot combustion gases entering at one end and leaving at the
others. The molten porous gray mass thus formed known as black ash is separated
from the calcium sludge and then crushed and leached with water in absence of
air in a series of iron tank.
The extract containing Na2CO3, NaOH, and other impurities is sprayed from
the top of a tower in counter current to flow of hot gases from the black ash
furnace. The sodium carbonate thus obtained is concentrated in open pans and
then cooled to get sodium carbonate. The product is calcined to get soda ash
which is re-crystallized to Na2CO3.10H2O. The sludge containing mostly CaS is left
behind as alkali waste.
The liquor remaining after removal of first batch of soda ash crystals is purified
and then causticized with lime to produce caustic soda.
Recovery of sulfur from alkali waste
Alkali waste is charged into cylindrical iron vessels arranged in series and CO 2
delivered from lime kilns is passed through it, the H2S gas thus obtained is then
conduced together with a regulated amount of air in a Claus kiln containing iron
oxide as catalyst. The exothermic reaction proceeds without further external heat.
Recovered sulfur is used in the manufacture of sulfuric acid.
2. Solvay's ammonia soda process
Raw materials
Basis: 1000kg sodium carbonate
Salt
= 1550kg
Limestone
= 1200kg
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Lecture: 9 Sodium carbonate
Dr. N. K. Patel
Coke
Ammonia as a catalyst
High pressure steam
Low pressure steam
Cooling water
Electric power
= 90kg
= 1.5kg (Loss)
= 1350kg
= 1600kg
= 40000 - 60000kg
= 210KWH
Sources of raw material
Common salt can be obtained from sea water, salt lake and sub –soil water
as described in Module: 3, Lecture: 8.
Lime stone is obtained from mineral calcite or aragonite, which can be used
after removal of clay, slit and sand (silica).
Reactions
CaCO3
CaO + CO2
C(s) + O2 (g)
CO2 (g)
CaO(s) + H2O (l)
Ca(OH)2 (aq)
NH3(aq) + H2O(l)
NH4OH(aq)
2NH4OH + CO2
(NH4)2CO3 + H2O
(NH4)2CO3 + CO2 + H2O
2NH4HCO3
NH4HCO3 + NaCl
NH4Cl + NaHCO3
2NaHCO3
Na2CO3 + CO2 + H2O
2NH4Cl + Ca(OH)2
2NH3 + CaCl2 + 2H2
ΔH = + 43.4kcals
ΔH = - 96.5kcals
ΔH = - 15.9kcals
ΔH = - 8.4kcals
ΔH = - 22.1kcals
ΔH = + 30.7kcals
ΔH = + 10.7kcals
Overall reaction
CaCO3 + 2NaCl
Na2CO3 + CaCl2
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Process equipment
Animation
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Dr. N. K. Patel
Ammonical
brine
Ammonia
absorbing tank
Gas outlet to absorbers
Brine Tank
Water
out
Cold water
Saturation tank
Water
out
Ca(OH2)
Ammonia recovery
tower
Carbonation tower
CaO Water
Cooling
coils
Water
in
NH3 (+ CO2)
CO2
Calcination
tower
Filter
pump
Vaccum
filter
Settling Tank
for ammonical brine
Na2CO3
Figure: Manufacturing of Sodium Carbonate by Solvay's Process
Ammonia is dissolved in a salt solution and ammoniate brine solution is
allowed to react with CO2 which is obtained by calcining lime stone with coke. A
precipitate of NaHCO3, thus obtain is then calcined to produce high purity Na2CO3.
Preparation and purification of brine
Saturated solution of NaCl is used. Brine contains impurities such as calcium,
magnesium and iron compounds. To remove calcium sulfate, magnesium and iron
salts sodium carbonate and sodium hydroxide are added. The precipitated
carbonates and hydroxide are removed by filtration. Sometimes sulfate are
removed with BaCl2 or the hot brine is treated with OH¯ and CO3-2 ions. The calcium,
magnesium and iron salts from saturated brine may be precipitated by dilute
ammonia and CO2 in a series of washing towers. The brine is purified by allowing it to
settle in vats, as a result of which precipitated CaCO3, MgCO3, Mg(OH)2 and iron
hydroxide settle down and pure brine solution is pumped to the ammonia absorber
tower, where it dissolve NH3 with the liberation of heat.
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Lecture: 9 Sodium carbonate
Dr. N. K. Patel
Ammoniation of brine
The purified brine is allowed to percolate down the ammonia tower in which
ammonia gas is passed through the bottom in a counter current fashion. The brine
solution thus takes up the necessary amount of ammonia and liberates heat. The
gas which escapes solution in the tank is absorbed by the brine falling down the
tower. Some carbon dioxide is also absorbed by ammonia, as a result of which
some insoluble carbonate is also precipitated. The ammoniated brine is allowed to
settle, coded to about 30°C and pumped to the carbonating tower.
Carbon dioxide formation
Limestone is calcined to get CO2 in a lime kiln filled with coke. As a result of
burning of coke necessary heat required for the decomposition of lime stone is
generated. CaO obtained from the lime kiln is converted into slaked lime and
pumped to the ammonia recovery tower.
Carbonation of ammonium brine
CO2 from the lime kiln is compressed and passed through the bottom of
carbonating tower down which ammoniated brine percolates. Carbonating towers
operated in series with several precipitation towers are constructed of cast iron
having 22-25meter height, 1.6-2.5meter in diameter. During the precipitation cycle,
the temperature is maintained about 20-25°C at the both ends and 45-55°C at the
middle by making use of cooling coils, provided at about 20ft above the bottom.
The tower gradually becomes flooded as sodium bicarbonate cakes on the cooling
coils and shelves. The cooling coils of the foulded tower are shut off. Then the fresh
hot ammoniated brine is fed down the tower in which NaHCO3 are dissolved to form
ammonium carbonate solution. The solution containing (NH4)2CO3, unconverted
NaHCO3 is allowed to fall down a second tower, called making tower. The making,
towers are constructed with a series of boxes and sloped baffles. Ammoniated brine
and CO2 gas (90-95%) from the bicarbonate calciner is recompressed and pumped
to the bottom of the making tower. The ammonium carbonate first reacts with CO2
to form ammonium bicarbonate and the latter reacting with salt, forms sodium
bicarbonate. The heat of exothermic reaction is removed by cooling coils.
Filtration
NaHCO3 slurry is then filtered on a rotary vacuum filter which helps in drying of
bicarbonate and in recovering ammonia. The filter cake after removal of salt and
NH4Cl by washing with water, sent to a centrifugal filter to remove the moisture or
calcined directly. During washing, about 10% NaHCO3 also passes into filtrate. The
filtrate containing NaCl, NH4Cl, NaHCO3 and NH4HCO3 is treated with lime obtained
from lime kiln to recover NH3 and CO2.
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Lecture: 9 Sodium carbonate
Dr. N. K. Patel
Calcination
NaHCO3 from the drum filter is calcined at about 200°C in a horizontal
calciner, which is either fired at feed end by gas or steam heated unit. The heating
being through the shell parallel to the product, which prevent the formation of
bicarbonate lumps.
The hot soda ash form the calciner is passed through a rotary cooler and
packed in bags. The exit gases (CO2, NH3, steam etc.) are cooled and condensed
to get liquid ammonia; the rich CO2 gas is cooled and returned to the carbonating
tower. The product from the calciner is light soda ash. To produce dense soda ash,
sufficient water is milled with it to form more mono hydrate Na2CO3.H2O and the
mixture is recycled.
Recovery of ammonia
The ammonia is recovered in strong ammonia liquor still, consisting of two
parts. The parts above and below the lime inlet is called as heater and lime still
respectively. The filtrate obtained from washing of NaHCO3 from the pressure type
rotary filter is fed into the heater, where free ammonia and carbon dioxide are
driven off by distillation. Dry lime or milk of lime (slaked lime) obtained from lime kiln is
fed through the lime inlet and mixed with the liquor from the heater. As the liquor
flows down the column, calcium chloride and calcium sulfate are formed and NH 3
gas is released.
NH4Cl + Ca(OH)2
(NH4)2SO4 + Ca(OH)2
CaCl2 + 2NH3 + H2O
CaSO4 + 2NH3 + 2H2O
The liquor from the bottom of the lime still is free from ammonia and contains
unreacted NaCl and largely CaCI 2, which is disposed off. The liquor is, therefore
allowed to settle in settling ponds and the clear liquid is evaporated till the salt
separates out and is sold as such for calcium chloride or further evaporated.
Kinetics and thermodynamics
The overall reaction shows that salt and calcium carbonate are the only raw
materials which are continuously supplied in the process and that produce sodium
carbonate and calcium chloride
CaCO3 + 2NaCl
Na2CO3 + CaCl2
Overall reaction of ammoniation of brine and then treatment of carbon
dioxide to ammoniated brine is as under
2NaCl + 2H2O +2NH3 + 2CO2
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Module: 3
Lecture: 9 Sodium carbonate
Dr. N. K. Patel
The above reaction shows the role of ammonia and carbon dioxide in the
process, and also determines the yield of the final product. There so conditions
favourable to it are precisely defined. For these the reaction is divided in to two
steps.
2NH3 + 2CO2 +2H2O
2NH4HCO3
---- (a)
2NaCl + 2NH4HCO3
2NaHCO3 + 2NH4Cl
---- (b)
Reaction (a) is undoubtedly favoured by low temperature because it requires
the dissolution of gas in water, is displaced to right by virtue of the fact that reaction
(b), which utilizes the product by subtracting it from (a) is displaced in the same
direction. Consequently, it is the precipitation of NaHCO3 according to (b) which is
the driving force behind the entire method.
The solubilities of the salts at various temperature is as under
Temperature
00C
200C
300C
NaCl
357
358.5
359
Solubility in gm/litre
NH4HCO3
NH4Cl
NaHCO3
120
298
69
217
374
95.4
269
467
109
Above data indicate that precipitation fortunately tends to take place
preferentially with satisfactory yields. On the basis of data and common ion effect
on precipitation of salts, physicochemical conditions most suitable for the forward
step of reaction (b) which causes precipitation of NaHCO3 are as under
To maintain lowest possible temperature in order to lower the solubility off
sodium bicarbonate
To maintain the greatest possible concentration of one or both the salts
appearing on the product side of reaction (b) with the aim of lowering still
further solubility of sodium bicarbonate.
These conditions are nevertheless discerningly applied because they serve to
bring about appreciable increase in the yields of NaHCO3 and permit the most
effective use of most costly reagent NH4HCO3 in reaction (b).
Attention is paid to the fact that, if precipitation temperature is always kept
low, the sodium bicarbonate separates in a microcrystalline form which is with
difficult to filter and it is soluble during subsequent washing on the filter, increase
requirement of NaCl.
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Lecture: 9 Sodium carbonate
Dr. N. K. Patel
Experimentally, the conditions which are most effectively reconcile the
physicochemical aspect of precipitation of sodium bicarbonate economically are
as under
284gm/liter (≈4.9mole/liter) of NaCl reacting with 76gm/liter (≈4.5mole/liter) of
NH3 instead of equimolecular solution of two reagents
Relatively high temperature (60-650C) at the start so as to allow the formation
of well-developed NaHCO3 crystallization seeds and increasing the volume of
these seeds to decrease the solubility of salt with gradual cooling.
Major engineering problem
Absorption units
The absorption units should be constructed to permit the downward travel of
growing sodium bicarbonate crystals. This is done by having each unit simulate a
very large single bubble cap with down sloping floors. The absorption is carried out in
towers filled with liquid. Hence CO2 must be compressed. Due to the compression
the partial pressure and solubility of CO2 increased at the end of carbonating cycle.
Making tower
Sodium bicarbonate formed in the making tower is drawn off as a suspension,
it is necessary to ensure that the precipitated sodium bicarbonate is easily filterable
and efficiently washable. It is carried out by regulating the temperature and
concentration in the making tower. During the precipitation cycle, the temperature
gradient is maintained at 200C at the both ends and 450C in the middle and fine
crystals of sodium bicarbonate are allowed to grow. The temperature is increased
from 200C to 45-550C by heat of reaction and reduced by using coils.
Development of suitable calcining equipment
Moist sodium bicarbonate will cake on sides of kiln, preventing effective heat
transfer through shell. Kiln must be equipped with heavy scraper chain inside and
wet filter cake must be mixed with dry product to avoid caking. These problems can
be avoided by using fluidized bed calciner.
Filtration unit
Filtration should be carried out by using vacuum on the drum filter. It helps in
drying the bicarbonate and in recovering ammonia
Ammonia recovery
Ammonia recovery costs 4-5 times that of Na2CO3 inventory so losses must be
kept low. By proper choice of equipment design and maintenance, losses are less
than 0.2% of recycle load or 0.5%/kg product or 1kg/ton of sodium carbonate.
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Lecture: 9 Sodium carbonate
Dr. N. K. Patel
Waste disposal
Large quantities of CaCl2-NaCl liquor is generated during the process. The
uses of these liquor is to be find out or dispose it as waste.
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Lecture: 9 Sodium carbonate
Dr. N. K. Patel
Module: 3
Lecture: 9
SODIUM CARBONATE
INTRODUCTION
Sodium carbonate (Na2CO3) also known as washing soda or soda ash, is a
sodium salt of carbonic acid. Most commonly occurs as a crystalline heptahydrate,
which readily effloresces to form a white powder, the monohydrate. Sodium
carbonate is domestically well known as a water softener. It can be extracted from
the ashes of many plants. It is synthetically produced in large quantities from salt and
limestone in a process known as the Solvay process.
Soda ash is the most important high tonnage, low cost, reasonably pure,
soluble alkali available to the industries as well to the laboratory.
MANUFACTURE
Sodium carbonate is manufactured by following process.
1.
2.
3.
4.
Leblanc process.
Solvay‘s ammonia soda process.
Dual process (modified Solvay‘s process)
Electrolytic process.
1. Leblanc process
The process has only historical importance, because is now been replaced
completely by Solvay process or modified by Solvay process.
Raw materials
Basis: 1000kg Sodium carbonate (98% yield)
Common salt
= 1126kg
Sulfuric acid
= 945kg
Lime stone
= 963kg
Coke
= 463kg
Sources of raw material
Common salt can be obtained from sea water, salt lake and sub –soil water
as described in Module: 3, Lecture: 8.
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Lecture: 9 Sodium carbonate
Dr. N. K. Patel
Sulfuric acid can be obtained by contact process as described in Module: 4,
Lecture: 18
Lime stone is obtained from mineral calcite or aragonite, which can be used
after removal of clay, slit and sand (silica).
Reactions
NaCl + H2SO4
NaHSO4 + NaCl
Na2SO4 + 4C
Na2S + CaCO3
NaHSO4 + HCl
Na2SO4 + HCl
Na2S + 4CO
Na2CO3 + CaS
(Black ash sludge)
CaS + H2O + CO2
CaCO3 + H2S
CaS + H2S
Ca(HS)2
Ca(HS)2 + CO2 + H2O
CaCO3 + 2H2S
H2S + O
H2O + S
Manufacture
Water
NaCl
Flue
gases
Lime
Stone Coke
NaCl
HCl
Salt
Cake
Furnace
Concentrated
H2SO4
Furnace
Black ash
Rotary
Furnace
Pulverizer
Mixer
Mixer
Crusher
Water
Water
out
Tower
Na2CO3
Leaching
Open pan
Evaporator
Calcination
tower
Water
in
Cooler
Hot gas
Figure: Manufacturing of Sodium Carbonate by Lablance process
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Lecture: 9 Sodium carbonate
Dr. N. K. Patel
Block diagram of manufacturing process
Diagram with process equipment
Animation
Common salt is first mixed with the conc. H2SO4 in equivalent quantities and
heated in a cast iron salt cake furnace by flue gases from adjacent coal of fire.
NaHSO4 along with HCl gas is formed. HCl is passed to tower packed with coke and
is absorbed through a spray of water comes down in the tower. The paste of
NaHSO4 is taken out and heated to a high temperature on the hearth of a furnace
along with some more common salt. NaHSO4 is thus converted into sodium sulfate,
known as salt cake.
The salt cake is broken or pulverized, mixed with coke and limestone and
charged into black ash rotary furnace consisting of refractory lined steel shells. The
mass is heated by hot combustion gases entering at one end and leaving at the
others. The molten porous gray mass thus formed known as black ash is separated
from the calcium sludge and then crushed and leached with water in absence of
air in a series of iron tank.
The extract containing Na2CO3, NaOH, and other impurities is sprayed from
the top of a tower in counter current to flow of hot gases from the black ash
furnace. The sodium carbonate thus obtained is concentrated in open pans and
then cooled to get sodium carbonate. The product is calcined to get soda ash
which is re-crystallized to Na2CO3.10H2O. The sludge containing mostly CaS is left
behind as alkali waste.
The liquor remaining after removal of first batch of soda ash crystals is purified
and then causticized with lime to produce caustic soda.
Recovery of sulfur from alkali waste
Alkali waste is charged into cylindrical iron vessels arranged in series and CO 2
delivered from lime kilns is passed through it, the H2S gas thus obtained is then
conduced together with a regulated amount of air in a Claus kiln containing iron
oxide as catalyst. The exothermic reaction proceeds without further external heat.
Recovered sulfur is used in the manufacture of sulfuric acid.
2. Solvay's ammonia soda process
Raw materials
Basis: 1000kg sodium carbonate
Salt
= 1550kg
Limestone
= 1200kg
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Module: 3
Lecture: 9 Sodium carbonate
Dr. N. K. Patel
Coke
Ammonia as a catalyst
High pressure steam
Low pressure steam
Cooling water
Electric power
= 90kg
= 1.5kg (Loss)
= 1350kg
= 1600kg
= 40000 - 60000kg
= 210KWH
Sources of raw material
Common salt can be obtained from sea water, salt lake and sub –soil water
as described in Module: 3, Lecture: 8.
Lime stone is obtained from mineral calcite or aragonite, which can be used
after removal of clay, slit and sand (silica).
Reactions
CaCO3
CaO + CO2
C(s) + O2 (g)
CO2 (g)
CaO(s) + H2O (l)
Ca(OH)2 (aq)
NH3(aq) + H2O(l)
NH4OH(aq)
2NH4OH + CO2
(NH4)2CO3 + H2O
(NH4)2CO3 + CO2 + H2O
2NH4HCO3
NH4HCO3 + NaCl
NH4Cl + NaHCO3
2NaHCO3
Na2CO3 + CO2 + H2O
2NH4Cl + Ca(OH)2
2NH3 + CaCl2 + 2H2
ΔH = + 43.4kcals
ΔH = - 96.5kcals
ΔH = - 15.9kcals
ΔH = - 8.4kcals
ΔH = - 22.1kcals
ΔH = + 30.7kcals
ΔH = + 10.7kcals
Overall reaction
CaCO3 + 2NaCl
Na2CO3 + CaCl2
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Process equipment
Animation
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Lecture: 9 Sodium carbonate
Dr. N. K. Patel
Ammonical
brine
Ammonia
absorbing tank
Gas outlet to absorbers
Brine Tank
Water
out
Cold water
Saturation tank
Water
out
Ca(OH2)
Ammonia recovery
tower
Carbonation tower
CaO Water
Cooling
coils
Water
in
NH3 (+ CO2)
CO2
Calcination
tower
Filter
pump
Vaccum
filter
Settling Tank
for ammonical brine
Na2CO3
Figure: Manufacturing of Sodium Carbonate by Solvay's Process
Ammonia is dissolved in a salt solution and ammoniate brine solution is
allowed to react with CO2 which is obtained by calcining lime stone with coke. A
precipitate of NaHCO3, thus obtain is then calcined to produce high purity Na2CO3.
Preparation and purification of brine
Saturated solution of NaCl is used. Brine contains impurities such as calcium,
magnesium and iron compounds. To remove calcium sulfate, magnesium and iron
salts sodium carbonate and sodium hydroxide are added. The precipitated
carbonates and hydroxide are removed by filtration. Sometimes sulfate are
removed with BaCl2 or the hot brine is treated with OH¯ and CO3-2 ions. The calcium,
magnesium and iron salts from saturated brine may be precipitated by dilute
ammonia and CO2 in a series of washing towers. The brine is purified by allowing it to
settle in vats, as a result of which precipitated CaCO3, MgCO3, Mg(OH)2 and iron
hydroxide settle down and pure brine solution is pumped to the ammonia absorber
tower, where it dissolve NH3 with the liberation of heat.
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Lecture: 9 Sodium carbonate
Dr. N. K. Patel
Ammoniation of brine
The purified brine is allowed to percolate down the ammonia tower in which
ammonia gas is passed through the bottom in a counter current fashion. The brine
solution thus takes up the necessary amount of ammonia and liberates heat. The
gas which escapes solution in the tank is absorbed by the brine falling down the
tower. Some carbon dioxide is also absorbed by ammonia, as a result of which
some insoluble carbonate is also precipitated. The ammoniated brine is allowed to
settle, coded to about 30°C and pumped to the carbonating tower.
Carbon dioxide formation
Limestone is calcined to get CO2 in a lime kiln filled with coke. As a result of
burning of coke necessary heat required for the decomposition of lime stone is
generated. CaO obtained from the lime kiln is converted into slaked lime and
pumped to the ammonia recovery tower.
Carbonation of ammonium brine
CO2 from the lime kiln is compressed and passed through the bottom of
carbonating tower down which ammoniated brine percolates. Carbonating towers
operated in series with several precipitation towers are constructed of cast iron
having 22-25meter height, 1.6-2.5meter in diameter. During the precipitation cycle,
the temperature is maintained about 20-25°C at the both ends and 45-55°C at the
middle by making use of cooling coils, provided at about 20ft above the bottom.
The tower gradually becomes flooded as sodium bicarbonate cakes on the cooling
coils and shelves. The cooling coils of the foulded tower are shut off. Then the fresh
hot ammoniated brine is fed down the tower in which NaHCO3 are dissolved to form
ammonium carbonate solution. The solution containing (NH4)2CO3, unconverted
NaHCO3 is allowed to fall down a second tower, called making tower. The making,
towers are constructed with a series of boxes and sloped baffles. Ammoniated brine
and CO2 gas (90-95%) from the bicarbonate calciner is recompressed and pumped
to the bottom of the making tower. The ammonium carbonate first reacts with CO2
to form ammonium bicarbonate and the latter reacting with salt, forms sodium
bicarbonate. The heat of exothermic reaction is removed by cooling coils.
Filtration
NaHCO3 slurry is then filtered on a rotary vacuum filter which helps in drying of
bicarbonate and in recovering ammonia. The filter cake after removal of salt and
NH4Cl by washing with water, sent to a centrifugal filter to remove the moisture or
calcined directly. During washing, about 10% NaHCO3 also passes into filtrate. The
filtrate containing NaCl, NH4Cl, NaHCO3 and NH4HCO3 is treated with lime obtained
from lime kiln to recover NH3 and CO2.
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Lecture: 9 Sodium carbonate
Dr. N. K. Patel
Calcination
NaHCO3 from the drum filter is calcined at about 200°C in a horizontal
calciner, which is either fired at feed end by gas or steam heated unit. The heating
being through the shell parallel to the product, which prevent the formation of
bicarbonate lumps.
The hot soda ash form the calciner is passed through a rotary cooler and
packed in bags. The exit gases (CO2, NH3, steam etc.) are cooled and condensed
to get liquid ammonia; the rich CO2 gas is cooled and returned to the carbonating
tower. The product from the calciner is light soda ash. To produce dense soda ash,
sufficient water is milled with it to form more mono hydrate Na2CO3.H2O and the
mixture is recycled.
Recovery of ammonia
The ammonia is recovered in strong ammonia liquor still, consisting of two
parts. The parts above and below the lime inlet is called as heater and lime still
respectively. The filtrate obtained from washing of NaHCO3 from the pressure type
rotary filter is fed into the heater, where free ammonia and carbon dioxide are
driven off by distillation. Dry lime or milk of lime (slaked lime) obtained from lime kiln is
fed through the lime inlet and mixed with the liquor from the heater. As the liquor
flows down the column, calcium chloride and calcium sulfate are formed and NH 3
gas is released.
NH4Cl + Ca(OH)2
(NH4)2SO4 + Ca(OH)2
CaCl2 + 2NH3 + H2O
CaSO4 + 2NH3 + 2H2O
The liquor from the bottom of the lime still is free from ammonia and contains
unreacted NaCl and largely CaCI 2, which is disposed off. The liquor is, therefore
allowed to settle in settling ponds and the clear liquid is evaporated till the salt
separates out and is sold as such for calcium chloride or further evaporated.
Kinetics and thermodynamics
The overall reaction shows that salt and calcium carbonate are the only raw
materials which are continuously supplied in the process and that produce sodium
carbonate and calcium chloride
CaCO3 + 2NaCl
Na2CO3 + CaCl2
Overall reaction of ammoniation of brine and then treatment of carbon
dioxide to ammoniated brine is as under
2NaCl + 2H2O +2NH3 + 2CO2
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2NaHCO3 + 2NH4Cl
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Module: 3
Lecture: 9 Sodium carbonate
Dr. N. K. Patel
The above reaction shows the role of ammonia and carbon dioxide in the
process, and also determines the yield of the final product. There so conditions
favourable to it are precisely defined. For these the reaction is divided in to two
steps.
2NH3 + 2CO2 +2H2O
2NH4HCO3
---- (a)
2NaCl + 2NH4HCO3
2NaHCO3 + 2NH4Cl
---- (b)
Reaction (a) is undoubtedly favoured by low temperature because it requires
the dissolution of gas in water, is displaced to right by virtue of the fact that reaction
(b), which utilizes the product by subtracting it from (a) is displaced in the same
direction. Consequently, it is the precipitation of NaHCO3 according to (b) which is
the driving force behind the entire method.
The solubilities of the salts at various temperature is as under
Temperature
00C
200C
300C
NaCl
357
358.5
359
Solubility in gm/litre
NH4HCO3
NH4Cl
NaHCO3
120
298
69
217
374
95.4
269
467
109
Above data indicate that precipitation fortunately tends to take place
preferentially with satisfactory yields. On the basis of data and common ion effect
on precipitation of salts, physicochemical conditions most suitable for the forward
step of reaction (b) which causes precipitation of NaHCO3 are as under
To maintain lowest possible temperature in order to lower the solubility off
sodium bicarbonate
To maintain the greatest possible concentration of one or both the salts
appearing on the product side of reaction (b) with the aim of lowering still
further solubility of sodium bicarbonate.
These conditions are nevertheless discerningly applied because they serve to
bring about appreciable increase in the yields of NaHCO3 and permit the most
effective use of most costly reagent NH4HCO3 in reaction (b).
Attention is paid to the fact that, if precipitation temperature is always kept
low, the sodium bicarbonate separates in a microcrystalline form which is with
difficult to filter and it is soluble during subsequent washing on the filter, increase
requirement of NaCl.
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Module: 3
Lecture: 9 Sodium carbonate
Dr. N. K. Patel
Experimentally, the conditions which are most effectively reconcile the
physicochemical aspect of precipitation of sodium bicarbonate economically are
as under
284gm/liter (≈4.9mole/liter) of NaCl reacting with 76gm/liter (≈4.5mole/liter) of
NH3 instead of equimolecular solution of two reagents
Relatively high temperature (60-650C) at the start so as to allow the formation
of well-developed NaHCO3 crystallization seeds and increasing the volume of
these seeds to decrease the solubility of salt with gradual cooling.
Major engineering problem
Absorption units
The absorption units should be constructed to permit the downward travel of
growing sodium bicarbonate crystals. This is done by having each unit simulate a
very large single bubble cap with down sloping floors. The absorption is carried out in
towers filled with liquid. Hence CO2 must be compressed. Due to the compression
the partial pressure and solubility of CO2 increased at the end of carbonating cycle.
Making tower
Sodium bicarbonate formed in the making tower is drawn off as a suspension,
it is necessary to ensure that the precipitated sodium bicarbonate is easily filterable
and efficiently washable. It is carried out by regulating the temperature and
concentration in the making tower. During the precipitation cycle, the temperature
gradient is maintained at 200C at the both ends and 450C in the middle and fine
crystals of sodium bicarbonate are allowed to grow. The temperature is increased
from 200C to 45-550C by heat of reaction and reduced by using coils.
Development of suitable calcining equipment
Moist sodium bicarbonate will cake on sides of kiln, preventing effective heat
transfer through shell. Kiln must be equipped with heavy scraper chain inside and
wet filter cake must be mixed with dry product to avoid caking. These problems can
be avoided by using fluidized bed calciner.
Filtration unit
Filtration should be carried out by using vacuum on the drum filter. It helps in
drying the bicarbonate and in recovering ammonia
Ammonia recovery
Ammonia recovery costs 4-5 times that of Na2CO3 inventory so losses must be
kept low. By proper choice of equipment design and maintenance, losses are less
than 0.2% of recycle load or 0.5%/kg product or 1kg/ton of sodium carbonate.
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Module: 3
Lecture: 9 Sodium carbonate
Dr. N. K. Patel
Waste disposal
Large quantities of CaCl2-NaCl liquor is generated during the process. The
uses of these liquor is to be find out or dispose it as waste.
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